Intake Manifold

What is an intake manifold and where is it located?

Every vehicle on the road today that is equipped with an internal combustion engine will have an intake (or inlet) manifold, which is located on the engine between the throttle body or carburetor and the cylinder head(s). The primary function of the intake manifold is to distribute the inlet air or air / fuel mixture to each of the cylinders evenly so that the engine can run properly and the vehicle can move. Since the early days of automotive engines, this main function has remained the same, however, as engines and fuel injection evolved, intake manifolds needed to as well.

The basic core design of all automotive intake manifolds has not changed; they consist of a plenum chamber and a number of individual runners, one runner per engine cylinder. Most carbureted and early fuel injected engines had intake manifolds constructed of cast iron. These were eventually replaced by cast aluminum because of the weight savings and better heat dissipation it offers over cast iron. The carburetor (or throttle body in the case of early fuel injection such as throttle body injection or central fuel injection) mounts to a flange on top of the plenum. Incoming air enters the carburetor and it is combined with atomized fuel. This air / fuel mixture then enters the plenum where it is distributed through the individual runners to the intake ports of the cylinder head. During the engine’s intake stroke, the intake valve(s) open, allowing this mixture to enter the combustion chamber where it is compressed and ignited, applying downward force to the piston. As the crankshaft rotates, the exhaust valve(s) opens and the piston moves back up the cylinder forcing these exhaust gasses through the exhaust valve(s) where it exits through the exhaust system. This series of strokes (intake, compression, combustion, and exhaust) is how all four-stroke internal combustion engines work.

The introduction of direct-port fuel injection (such as multi-port fuel injection and sequential-port fuel injection) allowed for increased power and efficiency. This fuel injection system incorporated individual fuel injectors (one for each cylinder) placed either towards the end of the intake manifold runner or on the cylinder head itself. Only inlet air travels through the throttle body, plenum, and into the runners. The atomized fuel is combined with this air close to the intake valve(s), which is much more efficient than the earlier designs. More and more vehicles produced today are equipped with a more advanced form of fuel injection known as direct injection, which moves the injector directly into the engine’s combustion chamber. Modern day inlet manifolds are usually made from aluminum or plastic composite materials.

How do I know my intake manifold needs to be replaced?

Intake manifolds fail by leaking, due to either cracking or warping. An intake air leak in the manifold will result in poor running conditions, such as an erratic or rough idle, hesitations, or sluggish acceleration. An intake leak will allow for additional unmetered air to enter the manifold. This is due to the fact that engines rely on the intake stroke of the engine to draw the air / fuel mixture into the engine, thus creating negative pressure (vacuum) within the intake manifold.

Many vehicles also have coolant passages in the intake manifold, meaning engine coolant travels through the manifold. Some applications that use plastic composite intake manifolds are prone to cracking at these passages.  Loss of coolant, overheating, or oil contamination can be signs of an intake manifold coolant leak. Many aftermarket intake manifolds have been redesigned with increased wall thicknesses to reduce the likelihood of cracking at these vulnerable areas. If you are experiencing any of these symptoms it is wise to do an inspection as you may need an intake manifold replacement.

Can I replace the intake manifold myself?

Replacing your vehicle’s intake manifold may often be easier than you think. On most cars and trucks, this can be done with basic hand tools. The difficulty of an intake manifold repair can range from vehicle to vehicle, and it will most likely be a very time consuming task. Typically, the air intake, throttle body, mass air flow sensor, fuel injectors, and spark plug wires will all need to be disconnected and removed. We recommend a quality repair manual which will take you through the process step-by-step and will contain the proper torque specifications and sequence.